In the manufacture and testing of medical devices, radial compression mechanisms are used to radially compress cylindrical devices such as stents, balloons, and catheters. For example, installation of a metal stent onto a catheter balloon is typically done by compressing the stent radially inward onto the balloon with enough pressure to permanently deform the stent to a smaller diameter and to slightly embed the metal stent into the plastic balloon. Further, a polymer stent can be installed onto a catheter balloon or a drug-coated stent (metallic or polymer) can be installed onto a catheter balloon by similar means as for a metallic stent. In a further example, a polymer catheter balloon is compressed radially after pleating to wrap the balloon tightly around the catheter shaft.
It is most often desirable to perform the compression process at a temperature above ambient, and furthermore to perform the compression process at some specific and well-controlled temperature. For example, the embedment of a compressed metallic stent into a catheter balloon, and therefore the stent's dislodgement force, can be improved if the balloon polymer is at an elevated temperature, because the balloon material is more deformable at the elevated temperature. Similarly, a polymer stent can be more reliably compressed onto a catheter balloon at elevated temperatures due to improved deformability of the stent material. In another example, the drug coating on a drug-eluting stent is rendered more compliant with elevated temperature, reducing risk of the coating cracking or delaminating from the stent when being crimped onto a balloon. In a further example, a polymer catheter balloon may be more tightly wrapped around a catheter shaft, with a smaller final diameter, after pleating if the compression is performed at elevated temperature.
Control of the elevated temperature is also important. Too low a temperature may result in poor stent dislodgement force, too-large wrapped balloon diameter, cracking of a polymer stent, or flaking of the drug coating on a drug-eluting stent during the compression process. Too high a temperature can cause damage to the elements (balloon, polymer stent, or a metallic stent may be over-compressed into its polymer balloon, or the drug compound in a drug-eluting coating on a stent or balloon may be damaged. Temperature uniformity along the length of each die and from die-to-die within the mechanism is also important. Products being crimped or compressed may require a narrow variance from the temperature setpoint, at any position along the product's length or circumference, to achieve acceptable quality.
One prior art device includes, for example, a radial compression mechanism wherein several wedge-shaped stainless steel or nickel alloy dies with planar surfaces are arranged around a common central axis to form a polygonal central cavity, the wedges being constrained and driven by a mechanism to control the size of the polygonal cavity. Prior art includes any of several types of radial compression mechanisms: such as the “hinged wedge” (U.S. Pat. No. 7,886,661); J-Crimp (U.S. Pat. No. 7,963,142); Twin-Cam (U.S. Pat. No. 8,245,559); and “Linear-motion Wedge” (U.S. Pat. No. 6,651,478), and U.S. Pat. Nos. 7,918,252, 7,407,377, 7,248,401, 7,308,748, and others.
In one example of prior art, each die of one of the above mechanism types is equipped with one or more electrically-driven heater elements to add thermal energy to heat the die, and at least one die is equipped with a temperature sensor (such as a thermocouple or RTD) to provide temperature feedback. This type of prior art is by far the most common, and is embodied by nearly all of the stent crimping and balloon wrapping machines sold. The most common material for catheter balloons is nylon, which has a glass transition temperature of about 45 to 50 deg. C. The die temperature for balloon wrapping or stent crimping of nylon balloons is typically in the range of 50 to 70 deg. C., in order to exceed the glass transition temperature and permanently deform the plastic. To allow processing of other materials, these mechanisms are typically controlled to temperatures ranging from 30 to 100 deg. C.
In another example of prior art, one of the above mechanism types is enclosed in a heated and temperature controlled chamber similar to an oven.
A shortcoming of the prior art is that it is impractical to cool the dies from some high temperature while maintaining the product under compression, because the cooling is much too slow. The control is one-sided, that is, there is no active cooling. Heat loss from the dies occurs only as small conduction losses at the mechanism interfaces and convective losses to the surrounding environment. In many applications, it is desirable to perform the compression process with the dies hot, then cool the dies to some significantly lower temperature with the product still under compression. In a typical stent-delivery balloon catheter, a metallic stent crimped onto a balloon catheter will exhibit reduced ‘rebound’ of the compressed balloon if it is cooled below the balloon's glass transition temperature prior to release from compression. In another example, wrapped and compressed catheter balloons typically “loosen” or “un fold” slightly upon relief of the compression and prior to cooling below the glass transition temperature. In-situ cooling of the balloon while under compression reduces this loosening. Without active cooling, this in-situ cooling takes a prohibitively long time, being therefore not productive.
Another shortcoming of the prior art results from the choice of the number of temperature sensors (one in the entire mechanism, one per die, or some distribution in between). A single temperature sensor is actually sensing only one die. Variations in individual die heat and variations in individual die heat loss results in temperature differences within the non-sensed dies compared to the controlled (sensed) die. Multiple temperature sensors within the die set have the effect of greatly increasing the complexity of control, as each sensor requires an independent control loop device.
A further shortcoming of this prior art mechanism is a lack of within-die temperature uniformity caused by a combination of localized heat input zones (the electrical heaters) and relatively poor thermal conductivity of the die material.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly it is an object of the present invention to provide new and improved radial compression mechanism. Another object of the present invention is to provide new and improved radial compression mechanism for compressing stents, catheters, balloons, polymer stents, drug-eluting stents, drug-eluting balloons, and the like in the medical device industry.
Another object of the present invention is to provide new and improved radial compression mechanism using actively-cooled and actively-heated dies to rapidly change die set temperatures.
Another object of the present invention is to provide new and improved radial compression mechanism using fluid forced-convection heat transfer in internal die ducts to effect rapid die heating and cooling.
Another object of the present invention is to provide new and improved radial compression mechanism with improved temperature control and uniformity within-die and within-mechanism.